He wasn’t a proven tax cheat back then. That is a new thing.
Trump Organiation found to have evaded NY State taxes. Why haven't the Feds acted?
We are talking about why nothing has been done up to and including the present day.
I seem to remember that New York has a special law they used for this that greatly expedited all of this, allowing criminal charges for things that are usually civil. The Feds don’t have this law, most likely.
How long has the IRS have since the NY announcement? How long do you think a audit takes? How about the Statute of Limitations? Were the issues raised unique to NY state tax law?